In the glassmaking industry, there is such a known process in which glass in viscous form is passed between two counterrotating rollers whose separation is the desired thickness of the sheet of glass which exits from between these rollers. The contact of the sheet with these rollers causes significant cooling of the sheet, at least on its surface. In fact, it is not possible to heat the rollers to the temperature of the glass to prevent its cooling, because such heating would cause the glass to stick to the rollers.
When the sheet of glass exiting from between the rollers is of a substantial thickness, the cooling due to the rollers is significant at the level of the "skin" of the sheet, and the glass then takes on a high viscosity in the vicinity of this skin, while the core of the sheet remains warmer and therefore less viscous. Subsequent cooling of this fluid core causes the core glass to shrink more than the cooler "skin" of the glass. This results in the development of stresses in the sheet, which can undesirably deform its structure, particularly when the sheet of glass is to be marked or imprinted with designs which must be positioned and executed with great precision.
When the sheet of glass exiting from the rollers is very thin (thickness less than 1 mm, for example), the cooling due to the rollers may extend through the entire thickness of the sheet. This sheet is then at too low a temperature to be taken into a subsequent operation of forming by pressing, for example. In addition, the minimal thickness of the sheet to be obtained at one time is limited to about 0.5 mm.
There are known processes for the manufacture of sheets of glass other than forming under pressure, such as the so-called "float glass" process or the processes known by the names of "fusion draw" or "down draw." The float glass process may involve unacceptable contamination with tin in certain applications. The other two are processes with a low production yield which also do not permit the use of glass which is easily devitrified. None of these three processes permits the production of sheets of glass with designs cut into the surface or through the entire thickness.
Currently, there is a need for a process for manufacturing glass sheets which can produce a shaped or structured sheet with great precision in its thickness and/or in the texture of its surfaces, this process also assuring a high-yield production compatible with manufacturing costs that are attainable for products intended for a mass market.